The present invention relates to methods for forming proppant-free channels in proppant packs in subterranean formation fractures.
Subterranean wells (such as hydrocarbon producing wells, water producing wells, and the like) are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a treatment fluid is pumped into a portion of a subterranean formation at a rate and pressure such that the subterranean formation breaks down and one or more fractures are formed. Typically, particulates, such as graded sand, are then deposited in the fractures. These particulate solids, or “proppant particulates” or “proppant,” serve to prevent the fractures from fully closing once the hydraulic pressure is removed. By keeping the fracture from fully closing, the proppant particulates aid in forming conductive paths through which fluids may flow.
Commonly used proppant particulates generally comprise substantially spherical particles, such as graded sand, bauxite, ceramics, or even nut hulls. Generally, the proppant particulates are placed in the fracture in a concentration such that they form a tight pack of particulates. Unfortunately, in such traditional operations, when fractures close upon the proppant particulates they can crush or become compacted, potentially forming non-permeable or low permeability masses within the fracture rather than desirable high permeability masses. Such low permeability masses may choke the flow path of the fluids within the formation. Furthermore, the proppant particulates may become embedded in particularly soft formations, negatively impacting production.
The degree of success of a fracturing operation depends, at least in part, upon fracture porosity and conductivity once the fracturing operation is stopped and production is begun. Traditional fracturing operations place a large volume of proppant particulates into a fracture and the porosity of the resultant packed propped fracture is then related to the interconnected interstitial spaces between the abutting proppant particulates. Thus, the resultant fracture porosity from a traditional fracturing operation is closely related to the strength of the placed proppant particulates (if the placed proppant crushes, then the pieces of broken proppant may plug the interstitial spaces) and the size and shape of the placed proppant (larger, more spherical proppant particulates generally yield increased interstitial spaces between the particulates).
One way proposed to combat problems inherent in tight proppant packs involves the placement of proppant aggregates comprised of multiple individual proppant particulates. The larger size of the proppant aggregates allows a reduced volume of proppant to be placed into the fracture while maintaining the structural integrity required to keep the fracture from closing and crushing the proppant aggregates. Additionally, the spaces between the proppant aggregates through which produced fluids may flow may be larger than the interstitial spaces that would be present between individual proppant particulates.
Another method proposed to combat problems inherent in tight spaces is to pump a substantially solids free fluid intermittently between pumping proppant particulates and/or proppant aggregates. The solids free fluid forms spaces within the proppant pack by preventing individual proppant particulates and/or proppant aggregates from gathering particularly close to one another. These spaces, or “proppant free channels,” form conductive channels through which produced fluids may flow. However, such intermittent pumping may be deleterious to operational equipment, as it requires the constant turning on and off of the equipment. Additionally, the intermittent pumping may cause additives in either the solids free fluid or other treatment fluids to settle out during the constant pressure changes (i.e., as the pumping equipment is stopped and begun again) and/or deposition of the additives in undesired locations in the subterranean formation.
Therefore, a method of forming proppant free channels in a proppant pack without causing equipment damage or negative effects on treatment fluids may be of benefit to one of ordinary skill in the art.